The present invention is related to sense amplifiers of semiconductor memory devices, and, in particular, to a sense amplifier and method of varying a reference level in a sense amplifier to be used for detecting data.
With the continued trend toward increased memory capacity, failures in reading data can occur due to relatively weak data signals and long delay times before bit line voltages swing to stable levels. For this reason, it is common for memory devices to employ sense amplifiers for amplifying the signal levels of data so as to provide more stable read operations, while requiring lower power levels by reducing the delay times of bit lines. An example technique for reading data by means of a sense amplifier is disclosed in U.S. Pat. No. 6,084,797, entitled “Method for reading a multi-level memory cell”, issued on Jul. 4, 2000 to Maloberti.
In general, a sense amplifier compares a core cell current Ic, that is detected from a core cell, with a reference cell current Ir that has a predetermined level during a read operation for the core cell. According to the comparison result, it is determined whether the sensed core cell is an on-cell or an off-cell. For example, if the core cell current Ic is less than the reference cell current Ir, the core cell is regarded as an off-cell D0. If the core cell current Ic is larger than the reference cell current Ir, the core cell is regarded as an on-cell D1. During this determination, the variations of core cell currents (Ion and Ioff; i.e., on-cell current and off-cell current) and a reference current (Iref; i.e., the reference cell current Ir), Ic and Ir, can be determined as follows, as a function of variation in power supply voltage.
FIG. 1 is a graphic diagram that demonstrates variation of the reference current and the core cell current as a function of varying power supply voltage in a semiconductor memory device.
As illustrated in FIG. 1, when a core cell is in an off-state (D0), a core cell off-current Ioff becomes smaller than the reference cell current Iref. However, the difference between the cell current and the reference current Iref is gradually reduced when a high power supply voltage is applied to the circuit based on the electrical characteristic of the core cell (refer to the shaded portion of the arrow 1 in FIG. 1). The core cell off-current Ioff is insufficient to enable a read operation by the sense amplifier due to the shortness of the sensing margin in comparison with the reference voltage Iref in the environment of high voltage HVcc. Otherwise, while the core cell on-current Ion is larger than the reference current Iref when the core cell is conditioned in an on-state (D1), the marginal difference from the reference current Iref is reduced at a low voltage condition LVcc by the electrical characteristic of the core cell (refer to the shaded portion of the arrow 2 in FIG. 1). Also, in this case, it is difficult for the sense amplifier to detect the on-state of the core cell.